This invention relates to hermetic sealing covers for enclosures or containers, and is more particularly directed to combination sealing covers or lids with incorporated sealing rings for packages for semiconductor devices.
Previous sealing covers and lids have been proposed to produce a package that is resistant to corrosion, especially in salt atmospheres. This is required to satisfy specifications of finished packages, such as Mil-Std-883C, Method 1009, Test Condition A, Salt Atmosphere (corrosion).
In the packaging of high reliability micro-circuits, a semiconductor ship is typically housed with a cavity formed in a ceramic housing and a lid is sealed to the housing to close the package. This lid is generally formed of an iron based metal called Kovar and the seal is formed by a pre-formed solder frame that is placed between the lid and package housing. In U.S. Pat. No. 3,946,190 there is described what is generally referred to as a combination cover wherein the solder frame is tack welded to the Kovar lid. The combination cover is capable of being hermetically sealed to the package to protect the enclosed chip.
The Kovar lid of a high reliability package is typically electroplated with a base coating of nickel and a final coating of pure gold. The nickel layer is usually about 100 micro inches thick while the gold outer layer is about 50 micro inches thick. The gold overlayer provides an oxide free surface for the solder which insures that the solder will uniformly wet the lid and thus produce the desired hermetic seal. It should be noted, however, that both coating layers are crystalline in form and thus contain cracks at the grain boundaries along which corrosive atmospheres can attack the iron base Kovar substrate. When this occurs the lid will rust and the protective coating will spall or flake away further promoting and accelerating the rusting process. This leads to premature failure of the package and degradation of the housed device.
Many high reliability packages are now expected to meet the requirements set out in the military specifications Mil-Std-883 C, Method 1009, Test Condition A, which is a salt atmosphere test. Under this test, the closure lids are exposed to a salt spray atmosphere under closely controlled climatic conditions. After twenty four hours to exposure to the corrosive atmosphere, the lids are washed and rinsed thoroughly and then inspected under a microscope to determine if the part has been adversely affected. A nickel-gold plated Kovar lid will invariably fail this test.
In an effort to adapt the Kovar lid to meet military specifications, a multi-coated structure has been devised wherein four coating layers rather than two are electroplated over the Kovar substrate.
One such lid is described in U.S. Pat. No. 4,601,958 to Levine. This lid is formed of a Kovar substrate (Kovar is an alloy of nickel iron and cobalt), covered with alternate layers of nickel and gold electroplate. The lid is affixed, using an 80/20 Au/Sn solder ring preform, to the semiconductor package. These lids are quite resistant to corrosion. However, because the Kovar base is corrosible, the entire lid must be completely covered with the Ni and Au plated layers. The nickel electroplate process is rather slow, which leads to long process times and hence high cost. Also, the two gold layers increase the expense of the lid considerably. Moreover, if a lid is defective and it is necessary to scrap the lid, the buried gold layer is not salvageable, so its gold value is lost.
The semiconductor industry has long sought a cover which would be extremely corrosion resistant, but could be easily manufactured with a minimum use of precious metals.